X-Ray Spectral Characterization of the Young Cygnus OB2 Population

Abstract

We analyze the X-ray spectra of the ∼8000 sources detected in the Cygnus OB2 Chandra Legacy Survey (this focus issue), with the goals of characterizing the coronal plasma of the young low-mass stars in the region and estimating their intrinsic X-ray luminosities. We adopt two different strategies for X-ray sources for which more or less than 20 photons were detected. For the brighter sample we fit the spectra with absorbed isothermal models. In order to limit uncertainties, for most of the fainter Cygnus OB2 members in this sample we constrain the spectral parameters to characteristic ranges defined from the brightest stars. For X-ray sources with <20 net photons we adopt a conversion factor from detected photon flux to intrinsic flux. This was defined, building on the results for the previous sample, as a function of the 20% quantile of the detected photon energy distributions, which we prove to also correlate well with extinction. We then use the X-ray extinction from the spectral fits to constrain the ratio between optical and X-ray extinction toward Cyg OB2, finding it consistent with standard “Galactic” values, when properly accounting for systematics. Finally, we exploit the large number of sources to constrain the average coronal abundances of several elements, through two different ensemble analyses of the X-ray spectra of low-mass Cyg OB2 members. We find the pattern of abundances to be largely consistent with that derived for the young stellar coronae in the Orion Nebula Cluster.

Optical and Infrared Counterparts of the X-Ray Sources Detected in the Chandra Cygnus OB2 Legacy Survey

“…Following the reddening vector, these sources can be stars associated with Cyg OB2 suffering large extinction, or low-mass members observed during a period of high X-ray activity. The analysis of their X-ray spectra and light curves will help us to better classify them (Flaccomio et al 2023). The sources matched only with the PM method populate mainly the region of the diagram with intermediate magnitudes.…”

“…The analysis of the X-ray properties of the X-ray sources with an OIR counterpart is beyond the scope of this paper, and it will be part of dedicated studies (Flaccomio et al 2023;Kashyap et al 2023). A brief comparison of the observed X-ray photons' energy between the X-ray sources with and without an OIR counterpart is shown in the next section.…”

The Chandra Cygnus OB2 Legacy Survey: Design and X-Ray Point-source Catalog

“…Optical and infrared counterparts to these sources are presented in Guarcello et al (2023), and an analysis of the likely Cyg OB2 members and contaminants is presented in Kashyap et al (2023). X-ray spectroscopy of the low- (Flaccomio et al 2023) and high-mass (Rauw et al 2015) members of the association is presented elsewhere, along with analysis and discussion.…”

“…While this test does not reveal periodicity or variability near the beginning or end of the observation and does not take into account variability in the background, it does provide a simple indication of potentially variable behavior that is useful to identify which sources are worthy of further investigation. This variability will be discussed in more detail in Flaccomio et al (2023), but we briefly summarize the single-parameter results here. We consider a source as being variable if the p-value of its light curve being reproduced by a source of constant flux is 0.005 (∼3σ confidence of variability).…”

“…Finally, in Section 5 we present an analysis of the quality of the final X-ray source catalog. Future papers will include X-ray spectral fitting of the brightest sources detected (Flaccomio et al 2023) and will combine this X-ray catalog with the available optical and infrared data covering the region to produce a fully multiwavelength catalog of Cyg OB2 members.…”